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High humidity in the house

Posted on 5/31/25 at 11:07 am
Posted by ThrowiTToTerrance
S. Carolina
Member since Oct 2007
590 posts
Posted on 5/31/25 at 11:07 am
2200 sq ft, one story ranch, 1970s built on a crawl space, wooded lot with afternoon sun on the house, otherwise shade.

2 year old roof, 2 year old HVAC Trane 4TTR4036N1 3ton, ridge vents running the full length of the roof, 15 soffit vents (6"x16") all replaced in last year and made sure air flow is good no insulation blockage, bathroom fans venting out thru roof vents.

struggling with 60-68% humidity even in early summer - no issues in winter or spring - stays in the 30-40%

Before replacing roof, we had a fan blowing air out - but with the ridge vents we didnt keep it - have had the hvac guy out and hes been zero help - we asked him about a whole home humidifier but he said that was terrible approach - I have noticed the attic is pretty humid with the wet bulb measurment (much more humid than the outside) and i keep the ridge vents clean of debris/leaves.

should i try more soffit vents? get a new HVAC guy out? any recommendations?



This post was edited on 5/31/25 at 11:11 am
Posted by SwampCollie
Louisiana
Member since Nov 2018
288 posts
Posted on 5/31/25 at 11:19 am to
Could be oversized AC.... which leads to short cycling and cooling without removing ample humidity.

68 is quite high....You could run a dehumidifier for some relief but it's a bandaid
Posted by CrawDude
Baton Rouge
Member since Apr 2019
5617 posts
Posted on 5/31/25 at 11:57 am to
A properly sized and operating HVAC should maintain near 50% humidity in summer with long run times. 3 tons for 2200 sq ft doesn’t seem oversized for a 1970s build. What temperature do you have your thermostat set at and how is humidity is being measured? Does your HVAC short cycle in cooling mode?

Posted by Jmcc64
alabama
Member since Apr 2021
1327 posts
Posted on 5/31/25 at 3:06 pm to
I don't know if it would help but does crawl space have 6 mil plastic covering the ground?

Curious what you have thermostat set at?

I assume it's hot enough for the cooling to be kicking on regularly, now.
Posted by Hamma1122
Member since Sep 2016
21189 posts
Posted on 5/31/25 at 6:16 pm to
Oversized a/c Not enough run time
Posted by DoctorO
BTR
Member since Jun 2010
284 posts
Posted on 5/31/25 at 7:36 pm to
Possibly poor air sealing in your air handler allowing humid air to be sucked in to the system.
Posted by diat150
Louisiana
Member since Jun 2005
46169 posts
Posted on 5/31/25 at 9:40 pm to
When did you notice this issue?
Posted by burgeman
Member since Jun 2008
10454 posts
Posted on 5/31/25 at 10:12 pm to
quote:

leads to short cycling


How long should an ac be running at a time?
Posted by LSUDad
Still on the move
Member since May 2004
60777 posts
Posted on 5/31/25 at 11:27 pm to
2,200 sq ft, 3 Ton, far from oversized. If the only change made, install another roof vent fan. Also, 15 soffit vents, seems low on a 2,200 sq ft home.
Posted by CrawDude
Baton Rouge
Member since Apr 2019
5617 posts
Posted on 5/31/25 at 11:31 pm to
quote:

How long should an ac be running at a time?


At design temperature (93 F for Baton Rouge) or higher a HVAC could run nearly non-stop from near noon until mid-late afternoon.

But from the article below.

“….how to tell if you have an oversized air conditioner. Just get your stop watch and time how long the AC runs on a hot afternoon. Ten minutes or less, and it’s definitely oversized. Twenty minutes would be OK. Thirty minutes at a time or longer, and your humidity levels should be fairly low.”

How to Tell if you have an Oversized Air Conditioner
Posted by baldona
Florida
Member since Feb 2016
22501 posts
Posted on 6/1/25 at 6:16 am to
Has the hvac guy offered to change your blower speed? I’m not an hvac pro but most hvac systems you can change your blower speed, they come with it at high speed as that’s most efficient for cooling. But lower speed allows more time for the air to travel over the coils and thus remove more moisture.

I’m not sure why he would say a dehumidifier is not worth it if the hvac isn’t doing the job. But you shouldn’t be hitting 68 routinely if it’s operating normally.
Posted by ruzil
Baton Rouge
Member since Feb 2012
17940 posts
Posted on 6/1/25 at 9:43 am to
quote:

Has the hvac guy offered to change your blower speed?


This is surely the place to start. Normally, it comes from the factory set at the medium setting. If the humidity is too high, it could be changed to the low setting so that it runs longer and dehumidifies the air inside the home.
Posted by CrawDude
Baton Rouge
Member since Apr 2019
5617 posts
Posted on 6/1/25 at 11:27 am to
quote:

This is surely the place to start. Normally, it comes from the factory set at the medium setting. If the humidity is too high, it could be changed to the low setting so that it runs longer and dehumidifies the air inside the home.

As I understand from reading professionals HVAC forums, a furnace blower motor from from the factory is usually set at the highest speed, an air handler lower motor might be different (there is a difference between a furnace & air handler).

In humid climates it recommended that blower motor speed be reduced to deliver 350 cfm air per ton (medium or medium high setting) from the default 400 cfm per ton (high speed setting) to improve dehumidication, and I gather that is not often done when HVAC systems are installed. However, slowing the furnace or air handler blower motor too much in cooling mode, e.g., slow speed, can often cause the evaporator coil to freeze.

So slowing blower motor fan speed is the easiest thing to try first to try to decrease humidity, but slowing it to much might cause the evaporator coil to freeze-up. The slowest fan speed is more likely to be used in heating mode.
This post was edited on 6/2/25 at 1:24 pm
Posted by GooseCreekMafia
Member since Jun 2017
906 posts
Posted on 6/1/25 at 12:42 pm to
My house started getting high humidity. Lasted about 3 or 4 days until I noticed the a/c never quit blowing. Someone had turned it to fan on instead of fan auto. Switched it back and cleared up with in a day.

Condensate was getting blown back into the house instead of draining out because the fan was always blowing
This post was edited on 6/1/25 at 12:43 pm
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